The invention relates generally to gas turbine engines, and, more specifically, to forming holes in gas turbine engine components.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in a high pressure turbine (HPT), which powers the compressor, and in a low pressure turbine (LPT), which powers a fan in a turbofan aircraft engine application, or powers an external shaft for marine and industrial applications.
Engine efficiency increases with temperature of combustion gases. However, the combustion gases heat the various components along their flowpath, which in turn requires cooling thereof to achieve an acceptably long engine lifetime. Typically, the hot gas path components are cooled by bleeding air from the compressor. This cooling process reduces engine efficiency, as the bled air is not used in the combustion process.
Gas turbine engine cooling art is mature and includes numerous patents for various aspects of cooling circuits and features in the various hot gas path components. For example, the combustor includes radially outer and inner liners, which require cooling during operation. Turbine nozzles include hollow vanes supported between outer and inner bands, which also require cooling. Turbine rotor blades are hollow and typically include cooling circuits therein, with the blades being surrounded by turbine shrouds, which also require cooling. The hot combustion gases are discharged through an exhaust which may also be lined and suitably cooled.
In all of these exemplary gas turbine engine components, thin walls of high strength superalloy metals are typically used to reduce component weight and minimize the need for cooling thereof. Various cooling circuits and features are tailored for these individual components in their corresponding environments in the engine. For example, a series of internal cooling passages, or serpentines, may be formed in a hot gas path component. A cooling fluid may be provided to the serpentines from a plenum, and the cooling fluid may flow through the passages, cooling the hot gas path component substrate and any associated coatings. Holes may be formed to access internal regions within the component.
For many newer hot gas path components, it may be desirable to form cooling holes after a coating has been deposited. If the coating is a ceramic, this basically eliminates using electric discharge machining (EDM) and similar machining techniques, as the ceramics typically are not electrically conductive. So for these applications, it would be desirable to use laser or abrasive liquid jet (ALJ) drilling to form the cooling holes. However, backstrike can be an issue with both of these techniques. For example, FIG. 5 of the present application illustrates a problem associated with forming the holes using ALJ. Namely, when ALJ drilling is used to make coolant supply holes into the interior cavities 114, once the ALJ punches through the substrate wall and defines the hole, the ALJ can also strike the interior surface of the opposite wall, thereby damaging that surface. Similar damage can occur with laser drilling.
It would therefore be desirable to provide backstrike protection to form cooling holes using laser or ALJ drilling.